The present invention relates to a surface illuminated photodiode and an optical receiver module on which the surface illuminated photodiode is mounted. More particularly, the invention relates to a surface illuminated photodiode appropriate for use in a module for optical communication and to an optical receiver module appropriate for use in an optical communication system.
There has been put into practice a p-i-n type photodiode (PIN-PD) as a device on the receiving side for optical transmission using an optical fiber. The PIN-PD is excellent in high frequency response as electrons and holes, which are generated due to incident light, move by strong drift field in an absorbing layer.
The basic structure of the PIN-PD is that an absorbing layer (I layer) is provided between a p-type cap layer and p-type contact layer, and an n-type contact layer. The principal of operation of the PIN-PD is that light incident into a reverse biased PD is converted to carriers (electrons and holes) in a depleted absorbing layer in which the holes spread and run to a p-electrode side and the electrons spread and run to an n-electrode side, and that the carrier is taken out as a current.
In the PIN-PD described in JP-A No. 32097/1996 and JP-A No. 82827/1993, a semiconductor material having an energy band gap larger than the absorbing layer is used for a p-type or n-type heavily doped layer.
In the technology described in JP-A No. 32097/1996 or JP-A No. 82827/1993, an accumulation (pile up) of carriers may occur due to heterojunction in which the energy band gap of the p-type or n-type semiconductor layer is larger than the absorbing layer (I layer). It is disadvantageous that high frequency response is degraded when the pile up occurs. To overcome this problem, there is also developed a mesa type PIN-PD having a p-type or n-type semiconductor layer formed using a heavily doped semiconductor material in which the energy band gap is equal to that of the absorbing layer or the difference therebetween is very small for the purpose of preventing the accumulation of carriers. However, such a PIN-PD has a problem that the high frequency response is degraded as slow carriers occur in the heavily doped layer when light incident into a light receiving section transmits through the absorbing layer and reaches the heavily doped layer on the side near the substrate. Although there may be a method of thinning the heavily doped layer to reduce the occurrence of such slow carriers, it is not easy to realize from the point of view of the etching accuracy and controllability of thickness in the formation of a mesa structure.